1. Field of the Invention
The present invention relates to an image scanning apparatus and a color determination method, and more particularly to an image scanning apparatus scanning a color original document while conveying the same.
2. Description of the Related Art
In recent years, Multi Function Peripherals (referred to as “MFP” hereinafter) that capture color image data by scanning an original document with three line sensors of red (R), green (G) and blue (B) are distributed. On the other hand, in order to scan a black-and-white original document, three data output from the three line sensors of red (R), green (G) and blue (B) are converted into one binary data or multi-value data to be processed. Therefore, Automatic Color Selection (ACS) function is provided to identify whether an original document is color or black-and-white.
On the other hand, some MFPs employ a scanning method of scanning an original document with three line sensors fixed in a main scanning direction by conveying the original document in a sub-scanning direction orthogonal to the main scanning direction. This MFP includes a transparent copyboard between the original document and the line sensor for keeping the distance between the original document and the line sensor constant at a scanning position. The light reflected by the original document is transmitted through the copyboard and received by the line sensor. Consequently, a problem arises when a foreign object is stuck on the copyboard and the line sensor scans the foreign object instead of the original document, thereby generating a streak of noise in the image data.
An image forming apparatus that prevents a streak of noise in image data is disclosed in Japanese Laid-Open Patent Publication No. 2002-344697. This conventional image forming apparatus includes an image scanning apparatus scanning an image while conveying an original document, which includes an exposure unit movable in such a manner as to reciprocate in the direction of conveyance of an original document and a control unit controlling the speed of conveyance of the original document and the speed of moving the exposure unit, characterized in that an image is scanned while an original document is conveyed and the exposure unit is moved. However, in-line correction allows three data output by three line sensors to be synchronized such that three data become data that are output by three line sensors respectively scanning the same portion of the original document. Therefore, if a foreign object exists on the copyboard, the foreign object is scanned at different positions of the original document respectively by three line sensors. Accordingly, in spite of a black-and-white original document, the portion where the foreign object is scanned becomes chromatic, so that the document is erroneously identified as a color document by ACS.